This invention relates to radio receivers, and more particularly, to a radio receiver capable of decoding a stereo transmission utilizing amplitude modulation.
Stereo broadcasting utilizing frequency modulation has been widely used for decades. In FM stereo broadcasting, a radio frequency carrier is frequency modulated with baseband information containing the left and the right (L+R) audio signals and by a subcarrier having a frequency well above the range of human hearing. This subcarrier is itself modulated by the difference between the left channel audio signal and the right channel audio signal (L-R). When the stereo broadcast is decoded, the L+R and L-R signals are recovered. By adding the L-R signal to the R+R signal, the left channel signal is obtained. Similarly, by subtracting the L-R signal from the L+R signal, the right channel signal is obtained.
There are several inherent disadvantages in FM stereo broadcasting. First, frequency modulation is employed which necessitates more complex transmitting and receiving equipment than is required for AM transmission and reception. Secondly, because the L-R subcarrier is required, a relatively large bandwidth FM signal must be transmitted, thereby minimizing the number of stations which may broadcast in the FM broadcast band. Thirdly, FM broadcasting is generally confined to the VHF regions and above, thereby minimizing the distance over which FM broadcasts may be received. Also, FM broadcasts are even more difficult to receive in automobiles due to the physical limitations placed on automobile radio antennas, as well as due to the terrain and similar effects provided by high-rise office buildings located in major cities.
For at least these reasons, in recent years much attention has been placed on the possibility of broadcasting stereo transmissions utilizing amplitude modulation within the standard AM broadcast band which extends from 550 Khz to 1610 Khz. A number of AM stereo systems have been proposed. It is highly desirable to broadcast AM stereo utilizing a system which allows standard monaural AM radio receivers to monaurally receive AM stereo broadcasts.
One such AM stereo system that has been proposed and is gaining widespread acceptance is the so-called "Kahn system". Various versions of the Kahn system are described, for example, in U.S. Pat. Nos. 3,218,393; 3,908,090; and 4,018,994 issued to Kahn, and which are hereby incorporated by reference. Mr. Kahn also described the Kahn system in an article entitled "A Stereophonic System for Amplitude-Modulated Broadcast Stations", IEEE Transactions on Broadcasting, Volume BC-17, No. 2, June 1971, pages 50-555, which is hereby incorporated by reference. The Kahn system is an AM stereo system which is compatible with existing AM broadcast transmission equipment and receivers. The transmitter provides a signal having an envelope which is modulated by the L+R signal. Thus, a standard monaural AM receiver can receive AM stereo transmissions broadcasted in accordance with the Kahn system and provide an audio output signal including left channel plus right channel information (L+R).
The L-R signal is used to phase modulate the carrier. A receiver constructed in accordance with the Kahn system decodes both the L+R signal and L-R signal and derives therefrom the left channel signal and the right channel signal.
FIG. 1 illustrates a transmitter 10 constructed in accordance with the Kahn system, which includes an input terminal 12 for receiving a left channel signal (L) and an input terminal 14 for receiving a right channel signal (R). Terminals 12 and 14 are coupled to a matrix circuit 16 which includes an adder 16a and a subtractor 16b. Adder 16a adds the left signal to the right signal and provides the resulting L+R signal on an output lead 18a. Similarly, subtractor 16b subtracts the right channel audio signal from the left channel audio signal and provides the resulting L-R signal on an output lead 18b.
The signals presented on output leads 18a and 18b are phase shifted by a 90.degree. difference network 20. Specifically, a first phase shifter 20a shifts the phase of the L+R signal by -45.degree., and places the resulting signal on an output lead 22a. Similarly, a second phase shifter 20b phase shifts the L-R signal by +45.degree. and places the resulting signal on an output lead 22b.
Output lead 22b, terminal 12, and terminal 14 are coupled to a second harmonic component circuit 24, the output lead of which is coupled to a summing circuit 26. Summing circuit 26 and circuit 24 modify the L-R signal to eliminate various undesired spectral components, and provide the modified L-R signal on an output terminal 28. This modified L-R signal is presented to a phase modulator 30 which uses the modified L-R signal to phase modulate a carrier signal provided by an oscillator 32. The output lead of phase modulator 30 is coupled to an amplitude modulator 34 which amplitude modulates the phase modulated carrier. Amplitude modulator 34 thus produces a signal having an envelope indicative of L+R information, while the carrier is phase modulated with a modified L-R signal. Since the envelope includes L+R information, a conventional AM receiver can be used to decode a monaural AM signal from the broadcast. Since L-R information is phase-encoded onto the carrier, a Kahn-type receiver can decode the L-R signal and derive therefrom separate left and right channel signals. The output lead of amplitude modulator 34 is presented to an RF power circuit 36 which translates the output of modulator 34 to a high power RF signal.
Transmitter 10 of FIG. 1 is described in greater detail in ISB AM Stereo Receiver Practices, published by Hazeltine Research, Inc., on Apr. 15, 1982, which is hereby incorporated by reference.
A receiver to be used in conjunction with transmitter 10 is also described in ISB AM Stereo Receiver Practices. The described receiver uses a pair of phase-locked loops to generate a reference IF signal. This reference IF signal is used to phase demodulate the received signal. Phase-locked loops employed in phase demodulation circuits generate harmonic interference signals, which causes noise and interference problems in AM stereo receivers.